In Schistosomiasis mechanisms involved in the progressive modulation of the granulomatous response to schistosome eggs were studied in vitro by using peripheral blood mononuclear cells from patients with early or chronic infection. Development of the granulomas required the presence of OKT4+ (helper) lymphocytes; OKT8+ (suppressor) cells inhibited granuloma formation. Patients with early infection had greater peri-oval granlomatous responses than did those with older infections. IgE responses of acutely infected patients, however, were less than those with chronic infection but the differences appeared mostly quantitative as qualitative analyses by immunoblotting techniques showed that essentially all allergens that were going to be recognized by a given individual were recognized very early. Experimental hookworm (Necator amercianus) infections in man have been established with a goal toward defining progressive changes in immunologic responses (especially IgE) to helminth infection.